What is the ruling on purchasing savings certificates with variable returns from Faisal Islamic Bank of Egypt, given the details on the bank's website?

We have no knowledge of the investment methods at Faisal Islamic Bank. For an investment to be permissible (halal), the conditions for lawful investment must be known. These include that the contract specifies the profit percentage, such as 5% or 10%, of the profits, not of the capital. If the Shariah-compliant conditions are met in this contract, then there is no harm in entering into it.